Actuators for power tool safety systems

ABSTRACT

Fast-acting and low-inertia actuators useable in various applications where a high force must be applied very quickly are disclosed. Power tools with detection systems configured to detect a dangerous condition between a person and a cutting tool are disclosed. In power tools, for example in a woodworking machine, a fast-acting and low-inertial actuator as disclosed herein can be used to retract a blade upon detection of a dangerous condition by a detection system. The actuator includes a charge of pressurized fluid and one or more electromagnets to selectively retain or release the pressurized fluid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/544,608, filed Aug. 19, 2019, which is a continuation of U.S. patentapplication Ser. No. 13/393,919, filed Mar. 2, 2012, which is acontinuation of International Application No. PCT/US10/002634, filedSep. 29, 2010, which claims the benefit of and priority from U.S.Provisional patent Application Ser. No. 61/278,111, filed Oct. 2, 2009.U.S. patent application Ser. No. 13/393,919 also claims the benefit ofand priority from U.S. Provisional Patent Application Ser. No.61/464,940, filed Mar. 11, 2011. All these applications are incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure relates to fast-acting and low-inertia actuators.More specifically, the disclosure relates to fast-acting and low-inertiaactuators useable in various applications where a high force must beapplied very quickly.

BACKGROUND ART

Safety systems for power tools are disclosed in InternationalApplication Number PCT/US00/26812, filed Sep. 29, 2000, and published asInternational Publication Number WO 01/26064 A2 on Apr. 12, 2001, thedisclosure of which is hereby incorporated by reference. Thatapplication claims priority to a number of priority documents, includingU.S. Provisional Patent Application Ser. No. 60/157,340, filed Oct. 1,1999. The safety systems disclosed in that application include adetection system adapted to detect a dangerous condition between aperson and a working portion of a machine, such as accidental contactwith the working portion, and a reaction system associated with thedetection system to cause a predetermined action to take place relativeto the working portion upon detection of the dangerous condition by thedetection system. The reaction system may be a retraction system toretract the working portion. Machines equipped with safety systems arealso disclosed.

Retraction systems for use in power equipment are also disclosed in U.S.Pat. No. 7,509,899, filed Aug. 13, 2001 and issued Mar. 31, 2009, andthat patent is also hereby incorporated by reference. The retractionsystems disclosed in that patent are configured to retract a cuttingtool at least partially away from a cutting region upon detection of adangerous condition by a detection system.

Power tool safety mechanisms are also disclosed in InternationalApplication Number PCT/US02/21790, filed Jul. 11, 2002, and published asInternational Publication Number WO 03/006213 A2 on Jan. 23, 2003. Thatapplication claims priority to a number of priority documents, includingU.S. Provisional Patent Application Ser. No. 60/304,614, filed Jul. 11,2001. That application discloses a sensing mechanism for detecting usercontact with an active portion of a power tool, and a system to rapidlydisplace the active portion away from a user extremity, oralternatively, to rapidly urge an extremity of the user away from theactive portion of the power tool.

Other retraction systems for power tools are disclosed in U.S. Pat. No.6,922,153, filed May 13, 2003 and issuing on Jul. 26, 2005. That patentdiscloses using a pyrotechnic and propellant to pivot a saw blade downor using a flywheel to do so.

A pyrotechnic drop mechanism for power tools is disclosed in U.S. Pat.No. 7,628,101, filed Oct. 30, 2006 and issued Dec. 8, 2009. That patentdiscloses using a pyrotechnic to move a piston and retract a saw blade.

At least as early as Mar. 19, 2010, a German company called Altendorfposted on the Internet an article describing a hand detection sensor foruse on wood processing machinery. That system uses a near-infraredsensor that purportedly can tell the difference between human skin andwood and other materials. Upon detection of human skin, a deflector orshutter stops the user's hand from reaching the saw blade.

German patent document DE 196 09 771 A1, with a publication date of Jun.4, 1998, describes a safety system for a circular saw bench in which aTheremin oscillator purportedly detects the proximity of a hand to ablade and a pneumatic cylinder then retracts the blade.

BRIEF DISCLOSURE

Fast-acting and low-inertia actuators are useable in safety systems forpower tools such as table saws, sliding table saws, jointers, up-cutsaws and other similar machinery. For example, the actuator may be usedto retract a blade or other cutting tool quickly to protect the useragainst serious injury if a dangerous or triggering condition isdetected, such as contact with or proximity to the blade or cutting toolby the user's body. The actuator may include a moveable and resettablepiston mechanically linked to the cutting tool so that upon actuation ofthe actuator, the piston moves to retract the blade or other cuttingtool. In certain embodiments of the actuators described herein, forceson the order of thousands of pounds (thousands to tens of thousands ofNewtons) can be applied within around 600 microseconds.

Generally, the embodiments of fast-acting, low-inertia actuatorsdescribed herein include a chamber of pressurized fluid (such as air)that is closed by a cap. The cap may be a piston head or a separatecomponent. The cap is held in place by an electromagnet. When theelectromagnet is turned off, the cap is released, allowing thepressurized fluid to escape and apply a force. The force of the escapingfluid may be used, for example, to move a piston and the movement of thepiston may perform a task, such as retracting a blade in a table saw. Inorder to achieve the desired speed and performance, the actuator must bedesigned to hold fluid at high pressure, the cap that opens to releasethe pressurized fluid must have low inertia relative to the forceapplied to it by the pressurized fluid while remaining sufficientlystrong and rigid to close the chamber and withstand the pressure withinthe chamber, the electromagnet must be sufficiently strong to retain thecap against the pressure of the fluid within the chamber, and theelectromagnet must be designed to turn off quickly without residualmagnetism slowing the movement of the cap, or in other words, themagnetic field produced by the electromagnet must terminate quickly.Fast-acting, low-inertia actuators and implementations of thoseactuators are illustrated in the attached figures.

A power tool as disclosed herein may include a cutting tool for cuttingworkpieces, a motor configured to drive the cutting tool, a detectionsystem configured to detect a dangerous condition between a person andthe cutting tool, support structure associated with the cutting tool andconfigured to allow the cutting tool to retract, and an actuator linkedto the support structure and adapted to retract the cutting tool upondetection of the dangerous condition by the detection system, where theactuator includes a charge of pressurized fluid.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a fast-acting, low-inertia actuator.

FIG. 2 shows a top view of the actuator of FIG. 1 .

FIG. 3 shows a sectional view of the actuator of FIG. 1 with a capclosed.

FIG. 4 shows a sectional view of the actuator of FIG. 1 with a cap open.

FIG. 5 shows a base of the actuator of FIG. 1 .

FIG. 6 shows a base and a conversion plate of the actuator of FIG. 1 .

FIG. 7 shows the top of a conversion plate.

FIG. 8 shows the bottom of a conversion plate.

FIG. 9 shows a base, conversion plate, and electromagnet of the actuatorof FIG. 1 .

FIG. 10 shows part of an electromagnet.

FIG. 11 shows a cross-section of parts of an electromagnet.

FIG. 12 shows an electromagnet.

FIG. 13 shows the bottom of an electromagnet.

FIG. 14 shows a base, conversion plate, electromagnet and cap of theactuator of FIG. 1 .

FIG. 15 shows a cap.

FIG. 16 shows an exploded view of a cap.

FIG. 17 shows grooves in the bottom of a cap.

FIG. 18 shows an actuator.

FIG. 19 shows a magnet keeper or housing.

FIG. 20 shows a top view of a magnet keeper or housing.

FIG. 21 shows a bottom view of a magnet keeper or housing.

FIG. 22 shows a coil used in an electromagnet.

FIG. 23 shows a schematic circuit.

FIG. 24 shows a piston cylinder.

FIG. 25 shows a piston and actuator.

FIG. 26 shows a piston.

FIG. 27 shows a cross-sectional view of an actuator and piston.

FIG. 28 shows a table saw.

FIG. 29 is a schematic drawing of a retraction system.

FIG. 30 is a schematic drawing of another retraction system.

FIG. 31 is still another schematic drawing of a retraction system.

FIG. 32 is yet another schematic drawing of a retraction system.

FIG. 33 is a schematic drawing of a retraction system using twoactuators.

FIG. 34 is a schematic drawing of a retraction system using a 4-barlinkage.

FIG. 35 is a schematic drawing of a retraction system using nestedspline shafts.

FIG. 36 shows a base of an actuator.

FIG. 37 also shows a base of an actuator.

FIG. 38 shows a base of an actuator with an air chamber.

FIG. 39 shows an actuator body.

FIG. 40 shows a magnet mount of an actuator.

FIG. 41 shows a bolt.

FIG. 42 shows an actuator body with a circuit board.

FIG. 43 shows a circuit board.

FIG. 44 also shows a circuit board.

FIG. 45 shows an actuator body with a foam layer.

FIG. 46 shows a foam layer.

FIG. 47 shows an actuator with three electromagnets.

FIG. 48 shows a plunger.

FIG. 49 also shows a plunger.

FIG. 50 shows a circuit board with three electromagnets.

FIG. 51 shows an electromagnet.

FIG. 52 shows an exploded view of an electromagnet.

FIG. 53 shows an electromagnet.

FIG. 54 also shows an electromagnet.

FIG. 55 shows a cross-sectional view of the electromagnet of FIG. 54 .

FIG. 56 shows an electromagnet housing and insert.

FIG. 57 shows an electromagnet insert.

FIG. 58 shows an electromagnet.

FIG. 59 shows a cross-sectional view of the electromagnet of FIG. 58 .

FIG. 60 shows an electromagnet.

FIG. 61 shows a cap.

FIG. 62 shows a contact disk.

FIG. 63 shows an actuator.

FIG. 64 shows a cap retainer.

FIG. 65 also shows a cap retainer.

FIG. 66 shows an actuator and piston.

FIG. 67 shows an actuator and piston head.

FIG. 68 also shows an actuator and piston head.

FIG. 69 shows an actuator and piston shaft.

FIG. 70 shows the base of an actuator.

FIG. 71 shows a fitting.

FIG. 72 also shows a fitting.

FIG. 73 shows a piston.

FIG. 74 shows an exploded view of the piston of FIG. 73 .

FIG. 75 shows a piston head.

FIG. 76 shows a knob.

FIG. 77 shows a system to provide pressurized air to a reservoir in anactuator.

FIG. 78 shows a flowchart illustrating various paths of cycling aretraction mechanism in a woodworking machine.

FIG. 79 shows an oil cylinder catch.

FIG. 80 shows an oil cylinder catch positioned adjacent a blade.

FIG. 81 shows a schematic circuit.

FIG. 82 shows the schematic circuit of FIG. 81 with a switch open.

FIG. 83 shows a map illustrating how the circuitry shown in FIGS. 84-95fits together.

FIG. 84 shows part of a circuit for controlling an electromagnet.

FIG. 85 shows another part of a circuit for controlling anelectromagnet.

FIG. 86 shows still another part of a circuit for controlling anelectromagnet.

FIG. 87 shows yet another part of a circuit for controlling anelectromagnet.

FIG. 88 shows a further part of a circuit for controlling anelectromagnet.

FIG. 89 shows more of a circuit for controlling an electromagnet.

FIG. 90 shows still more of a circuit for controlling an electromagnet.

FIG. 91 shows yet more of a circuit for controlling an electromagnet.

FIG. 92 shows an additional part of a circuit for controlling anelectromagnet.

FIG. 93 shows another additional part of a circuit for controlling anelectromagnet.

FIG. 94 shows yet another additional part of a circuit for controllingan electromagnet.

FIG. 95 shows still another additional part of a circuit for controllingan electromagnet.

FIG. 96 shows a simplified diagram of a circuit controlling oneelectromagnet.

FIG. 97 shows a simplified diagram of a circuit controlling threeelectromagnets.

DETAILED DESCRIPTION AND BEST MODE(S) OF THE DISCLOSURE

FIGS. 1 and 2 show an actuator 10 which has a base 12, a conversionplate 14, a magnet keeper 16, and a cap 18. Cross-sectional views ofactuator 10, taken along the line A-A in FIG. 2 , are shown in FIGS. 3and 4 . FIG. 3 shows a cross-sectional view with the cap closing achamber, and FIG. 4 shows a cross-sectional view with the cap raised andthe chamber open.

Base 12 includes a hole or channel 20 through what may be thought of asthe front side of the base, as shown in FIG. 5 . Channel 20 opens into acylindrical fluid or air chamber 22 in the center of the base. Hole 20serves as an inlet through which air is pumped into the air chamber byan air compressor. The top of the air chamber is open and is surroundedby four holes 26 which will be used to attach the base to the rest ofthe assembly. Base 12 also includes four holes 27 which may be used toattach a cylinder to the base, as will be described below.

A conversion plate 14, shown in FIGS. 6-8 , fits over base 12 and helpsenclose the air chamber. The conversion plate has a hole 28 in thecenter and four holes 30 that line up with holes 26 in the base. Thebottom of the conversion plate has a cylindrical recessed area 32 thatis positioned over air chamber 22 in the base but has a slightly smallerdiameter than air chamber 22, as shown in FIG. 4 . Recessed area 32 issurrounded by flat surface 34. A large o-ring fits into a circulargroove 36 in the base and extends around air chamber 22. When theconversion plate is attached to the base, the o-ring in groove 36contacts flat surface 34 of the conversion plate and seals air chamber22 at the interface. Two small holes 38 are drilled through both thebase and conversion plate outside of the air chamber and are used topass electrical wires to and from the interior of the actuator, as willbe described below.

Conversion plate 14 also has two small holes 40 closely surrounding thecenter hole 28. These two holes are used to mount an electromagnet 42,as shown in FIG. 9 . Electromagnet 42 is composed of a first steelcylinder 43 with a hole 44 through the center, as shown in FIGS. 10 and11 . A second steel cylinder 45 is positioned concentrically over andaround the upper portion of first cylinder 43 and is sized to create agap or space 46 between the outer surface of the first cylinder and theinner surface of the second cylinder. Second steel cylinder 45 can beheld in place by a press fit, threads, welding, gluing, screws, etc.

A coil 52 of copper wire is wound around the upper portion of firstcylinder 43 and second cylinder 45 is placed over the coil so the coilfills gap 46, as shown in FIGS. 9 and 12 . The copper wire forming coil52 has two ends 54 and those ends thread down through holes 56 in firstcylinder 43 and out the bottom of each side of the electromagnet, oneend through each hole. There is a recessed area 58 at the bottom of theelectromagnet below each hole to leave space for the wire to exit thebottom of the electromagnet. The wires are routed around the outside ofelectromagnet 42 and through holes 38 in the conversion plate and baseso they may be connected to an electric circuit to control theelectromagnet.

Two screws extend up through holes 40 in the conversion plate intothreaded holes 62 in the bottom of the electromagnet to attach theelectromagnet to the conversion plate. Screw holes 40 are encircled bygrooves 60 on the top of the conversion plate, and an o-ring fits intoeach groove to seal the air chamber. Similarly, groove 68 encircles hole28 and another o-ring fits into that groove to further seal the airchamber at the interface between the conversion plate and theelectromagnet.

Hole 44 through first cylinder 43 is an extension of air chamber 22 inbase plate 12, as shown in FIG. 4 , and hole 44 extends up through thecenter of electromagnet 42. As shown in FIG. 14 , cap 18 is positionedon top of the electromagnet and covers hole 44. When the electromagnetis turned on, the cap is magnetically attracted to the electromagnet andseals air chamber 22 and hole 44. Cap 18 a flat, somewhat thin, circularpiece of steel with a rounded edge 70 around the perimeter that slopesdownward so that the bottom surface of the cap has a larger diameterthan the top, as shown in FIG. 15 . A hole 80 is in the center of thecap, and a screw 82 passes through the hole and into a correspondinghole 84 in the center of a guide 86 to attach the guide to the bottomsurface of the cap, as shown in FIG. 16 . In the depicted embodiment,the guide is a plastic part with a flat, round top surface and with hole84 in the center. Three flat, long fins 88 extend down below the top.The fins help keep the cap from tilting when it moves up or down andthey are long enough that about three-quarters of the length of the finsremains in hole 44 when the cap is fully raised. The fins are arrangedso their outer edges are vertical and separated by 120 degrees. Thebottom of each fin is cut at an angle so that the outside edge is longerthan the inside edge. This minimizes the weight of the guide.

The bottom of the cap has two circular grooves 72 and 74, as shown inFIG. 17 , and those grooves are concentric with the center of the cap.O-rings are fitted into the grooves and the o-ring in outer groove 72seals against first cylinder 43 of the electromagnet when the cap isdown. The o-ring in inner groove 74 seals against the top of guide 86around hole 80 in the center of the cap. The top of the guide is of adiameter such that it overlaps the o-ring in outer groove 72 overapproximately half of the width of the o-ring to hold the o-ring inplace. Guide 86 is used to keep the cap aligned with hole 44 so that asthe cap moves up and down, the o-ring in groove 72 will always alignwith the edge of hole 44 so that the air chamber can be sealed reliably.

A magnet keeper 16, shown in FIGS. 18-21 , fits over the electromagnetand conversion plate. The magnet keeper is a generally cylindricalhousing with a hole 90 through the top center. Hole 90 is large enoughto fit over cap 18 with sufficient clearance for the cap to move up anddown. A shallow cutout 92 on the bottom of the magnet keeper, which isconcentric with but has a slightly larger diameter than hole 90, allowssome room around the bottom of the magnet for the two ends 54 of thewire from the electromagnet. Cut out 92 joins another recessed area 94which extends over the two holes 38 in the conversion plate and baseplate through which the two ends of wire 54 are routed. The magnetkeeper is mounted over the conversion plate and to the base plate byscrews that extend up through holes 26 in the base and holes 30 in theconversion plate to thread into holes 96 in the magnet keeper.

As shown in FIGS. 18, 19 and 20 , the top of the magnet keeper isdesigned to allow air from the air chamber to escape when the cap isreleased, while also providing abutments to stop the upward movement ofthe cap. Four equally spaced ducts or air outlets 100 are formed aroundhole 90. Air outlets 100 consist of cut outs with slanted bottoms androunded corners that slant down and inward toward the perimeter of hole90. Between the air outlets, the top surface of the magnet keeperextends towards the center with a decreasing thickness forming shouldersor fingers 102. The fingers 102 are curved underneath to match thecurvature along the top edge of the cap. The four fingers 102 extendtoward and slightly over the rounded edge of cap 18 to form abutments orcatches to stop the upward movement of the cap. In this embodiment,fingers 102 allow the cap to move upward about one quarter of an inch.The disclosed embodiment uses four channels and four fingers to helpkeep the cap from tilting when it abuts the fingers, although more orless fingers and channels could be used. When the cap is raised, the topsurface of the cap is flush with the top surface of the magnet keeper.

FIG. 22 shows a cross-sectional view of electromagnet 42 and helpsillustrate how the electromagnet works to hold and release cap 18. Inthis figure, the wire loops forming coil 52 are represented by dots andXs. In use, the coil is connected to a voltage source so that currentflows in the wire and around the coil. The dots in FIG. 22 representcurrent flowing out from the page on the left side of the coil and theXs represent current flowing into the page on the right side of thecoil. That current flow creates a magnetic field that points up insidethe coil, wraps around the top of the coil, and then comes back down onthe outside of the coil. With the cap in place, the magnetic field isable to trace a looped path within metal creating a closed-loop magneticcircuit, as represented by arrows 150 in FIG. 22 , which makes themagnetic field much stronger than if there were a non-metal gap that themagnetic field had to travel across to complete a loop.

Generally, an electromagnet creating as much holding force as possibleis desired so that the air chamber can be pressurized as high aspossible. However, physically larger electromagnets that create higherholding forces also release the cap slower because the residual magneticfield takes time to dissipate after the current passing through the coilis turned off. This is in part due to the inductance of the coil and toeddy currents generated in the metal supporting and surrounding the coilwhen the coil current is shut off. Accordingly, the electromagnet mustbe designed to achieve a desired balance of holding force and releasetime. Electromagnets creating approximately 50 to several 1000 lbs offorce may be designed for different applications. In some of theapplications described subsequently concerning table saws, one or moreelectromagnets, each having a diameter of approximately 2.5 inches, acoil having on the order of hundreds of loops, and an electric currenton the order of 1 amp or less, is sufficient to generate a holding forceon the order of hundreds of pounds each. Smaller electromagnets,however, will have smaller fluid outlets and thereby will be limited inthe volume of pressurized fluid that can be released quickly to performa task. Using multiple small electromagnets is one way to overcome thatlimitation because multiple electromagnets will have multiple outlets.For any size of electromagnet, using high resistance steel or other highresistance metal for the electromagnet causes eddy currents to dissipatemore quickly due to the higher resistance, and thereby minimizes thetime to release the cap. Also, to maximize the holding force of theelectromagnet, sufficient electric current should be supplied to thecoil to generate enough magnetic field to saturate the metal surroundingthe coil. Additionally, electromagnets with double coils or multiplecoils could be used.

In order to decrease the release time of cap 18, a current can be forcedthrough the coil in the opposite direction for a brief amount of time tospeed the decay of the dissipating magnetic field. The counteracting or“turn off” current is only applied for a brief amount of time becauseotherwise it will begin to re-magnetize the magnet with the magneticfield pointing in the opposite direction.

FIG. 23 shows conceptually how to energize the electromagnet and providea current in the opposite direction to speed the decay of thedissipating magnetic field. In this figure, the electromagnet isrepresented as an inductor and V1 is the voltage source that suppliesthe current to the electromagnet. Closing the “Hold” switch turns theelectromagnet on to hold cap 18 in place. Opening the “Hold” switchcauses the magnetic field to dissipate and release cap 18. Closing the“Release” switch at the same time the “Hold” switch is opened connects acapacitor charged to a high voltage, labeled “Pulse Cap,” to theelectromagnet and provides a pulse of current in the opposite directionto speed the decay of the dissipating magnetic field to more quicklyrelease cap 18. By way of example, 100 to 1,000 volts can be applied tocause the reverse current and induce large reverse eddy currents.Capacitances ranging from 1 microfarad to 100 microfarads are typicalvalues for the capacitor, depending on the magnet and other factors.

In addition to reversing the current in the electromagnet, the magneticfield can also be more quickly dissipating by constructing theelectromagnet core out of laminated strips of metal layered togethersuch that there are a series of non-metallic interruptions in the innerand outer cylinders perpendicular to the direction of the current. Thelaminations interrupt the metal so that large eddy current loops cannotform.

An actuator as described above may be used to quickly accelerate apiston. In FIGS. 24 and 25 , actuator 10 is shown incorporated in apiston system where the base 12 of the actuator forms the base of thepiston system. The bottom of a large cylinder 160 fits in a groove 162on the top of base 12. The top of the cylinder fits into a similargroove in a cover 164 which covers the top of the cylinder. Four rods orlong bolts, such as bolt 166, connect base 12 to cover 164 to holdcylinder 160 in place. The bolts pass through holes 27 in the base andthread into corresponding holes in the cover.

Cover 164 also includes a hole 168 through which a piston rod 170extends. A piston head 172 is attached to the other end of rod 170within cylinder 160. The piston head is a round, flat piece thick enoughto be solid and resist warping. There is a groove 174 around the side ofthe piston head, as shown in FIG. 26 , and an o-ring fits into thegroove. The o-ring contacts the inner side of cylinder 160 to create aseal between the piston head and the cylinder so that air or gasexpanding under the piston head does not escape.

FIG. 27 shows a cross-sectional view of actuator 10 and the pistonsystem described above. In this figure, air chamber 22 and hole 44 aresealed by cap 18, and piston head 172 rests on the top of magnet keeper16. To operate the piston, the electromagnet is turned on to sealchamber 22 and the chamber is filled with air through the inlet 20 atthe front of the base of the actuator. This may be done with an aircompressor that is followed by one or more boosters to get the airpressure up to the desired level. To fire the piston, the electromagnetis turned off so that cap 18 moves upward releasing air from chamber 22through outlets 100 at the top of the magnet keeper. The air expandsunder the piston head driving the piston upward. In an alternativeembodiment, the piston head may include a portion that extends below thetop surface of the magnet keeper and toward the cap (or the cap mayinclude a portion that extends up toward the piston head) so that whenthe cap moves up, the cap strikes the piston head and imparts at leastsome of its energy to the piston head before the cap is stopped by themagnet keeper.

The volume of chamber 22 is sized according to the volume that is to befilled under the piston head once the air is released. This, in turn,will depend on the mass of the piston, the mass of the load the pistonis to move and the velocity the piston is to acquire in a certaindistance. A chamber filled with a charge of air at a desired pressurebut which is too small will not be able to supply enough air to move aheavy load far enough for it to reach an expected velocity under theacceleration imparted to it by the expanding gas. The combinedcross-sectional area of the four outlets through which the air isreleased is sized according to how fast the air is to be released sincethere is a limit as to how fast air can move and if the outlets are toosmall, the airflow will be restricted.

The idea of a charge of fluid stored locally or near the piston issignificant when the actuator is used in a safety system for powerequipment. A remote source of air cannot be relied upon in thatsituation because the relatively slow speed of sound in air of about 1foot per millisecond means it would take too long for a remote charge ofpressurized air to reach the piston and start it moving. A person couldbe seriously injured in the time it took for the remote charge ofpressurized air to reach the piston. A charge of pressurized air nearthe piston provides the fastest response; one cannot achieve the sameresults with a remote charge. Charges of pressurized fluid may belocated as close as possible to the piston and/or cutting tool. Forexample, pressurized charges may be located less than one meter or lessthan one half meter from the cutting tool. Charges of pressurized fluidmay be located within a housing enclosing the structure supporting thecutting tool in a piece of power equipment, such as within the cabinetof a table saw.

Once the air is released and the piston is driven upward under the forceof the expanding gas, the air in the volume in the cylinder above thepiston will become pressurized and cause the piston to decelerate. Theheight of the cylinder is sized according to how far the piston is to bemoved and how soon it is to decelerate. It is desirable to decelerate apiston that is moving fast so that it does not jolt and potentiallydamage other parts connected to the piston. Ideally, both theacceleration and the deceleration of the piston would be constant. Theair above the piston forms an air spring which helps to decelerate thepiston but the piston can be decelerated even more quickly and moreevenly after the piston has reached a desired height by adding one ormore pressure outlets to the side of cylinder 160, such as pressureoutlet 180 shown in FIG. 24 . The pressure outlets arepressure-sensitive valves that open to allow air below the piston headto be escape. They are positioned on the cylinder at a height just abovethe desired height the piston is to reach under the pressure of theexpanding gas so that as soon as the piston passes the pressure outletsthe air under the piston head is allowed to escape through the pressureoutlets thus aiding the deceleration of the piston.

An actuator and piston assembly as described above can be used in asafety system for various power tools, including a table saw. A tablesaw is a woodworking tool that includes a table and a circular bladethat extends up through the table, as shown in FIG. 28 . Table saws aredescribed in U.S. Patent Publications 2007/01514330A1 and 2010/0050843A1and in U.S. Pat. No. 7,707,920, which are incorporated herein byreference. A piece of wood, or other material to be cut, is placed onthe table and pushed into contact with the spinning blade to make a cut.Unfortunately, it is common for users of table saw to accidentallycontact the spinning blade and be seriously injured. To address theseinjuries, the saw or other power tool can be equipped with a system todetect contact with or dangerous proximity to the blade, as has beendescribed in various patents, including but not limited to U.S. Pat.Nos. 7,055,417, 7,210,383, 7,284,467, and 7,600,455, all of which areincorporated herein by reference. Upon detection of such a dangerouscondition, an actuator and piston assembly as described herein can betriggered to retract the blade quickly to mitigate any injury. A systemto retract a blade is particularly applicable in machines where theblade or cutting tool has substantial inertia making it difficult tostop the blade quickly.

FIG. 29 shows one example of how an actuator and piston assembly can beimplemented to retract a spinning blade in a table saw. In FIG. 29 , acircular blade 400 is mounted on a rotating shaft or arbor 420. Thearbor, in turn, is supported by an arbor support 422, and the arborsupport is mounted in the machine so that it can pivot around point 424.A detection system 220 is adapted to detect a user coming into contactwith blade 400.

The detection system includes a sensor assembly, such as contactdetection plates 440 and 460, capacitively coupled to blade 400 todetect any contact between the user's body and the blade. Typically, theblade, or some larger portion of the machine is electrically isolatedand the detection system imparts an electrical signal to the bladethrough detection plate 440 and monitors that signal for changesindicative of contact through detection plate 460. The detection systemtransmits a signal to a control system 260 when contact between the userand the blade is detected.

Control system 260 includes one or more instruments 480 that areoperable by a user to control the motion of blade 400. Instruments 480may include start/stop switches, speed controls, direction controls,light-emitting diodes, etc. Control system 260 also includes a logiccontroller 500 connected to receive the user's inputs via instruments480. Logic controller 500 is also connected to receive a contactdetection signal from detection subsystem 220. Further, the logiccontroller may be configured to receive inputs from other sources (notshown) such as blade motion sensors, work piece sensors, etc. In anyevent, the logic controller is configured to control operation of thesaw in response to the user's inputs through instruments 480. However,upon receipt of a contact detection signal from detection subsystem 220,the logic controller overrides the control inputs from the user andtriggers an actuator and piston assembly to retract the blade. Anexemplary control system for a fast-acting safety system is disclosed inU.S. Pat. No. 7,600,455, which is incorporated herein by reference.

In FIG. 29 an actuator and piston assembly is shown at 800 connected tologic controller 500 by line 502. A piston rod 600 is mechanicallycoupled to arbor support 422 so that when the actuator is fired, thepiston rod pulls the arbor support downward to retract the blade. Inthis embodiment, two electromagnets in one actuator are used to drivethe piston. Alternatively, two actuators could be used. In FIG. 29 ,piston rod 600 extends past two electromagnets and through a reservoir,and the actuator is configured to pull the piston rod down.

Once the actuator has fired, it can be reset by turning on theelectromagnet to close the cap and re-charging the air chamber. A springor other mechanism can be used to move the piston back against themagnet keeper and cap. A spring can also be used to bias the cap to aposition where the air chamber is closed. An actuator as describedherein can be cycled (i.e., triggered and reset) repeatedly. This is asignificant feature because it allows a system using the actuator, suchas a safety system in woodworking equipment, to be repeatedly triggeredwithout a user having to replace parts of the safety system.

Other examples of how an actuator and piston assembly can be implementedto retract a spinning blade in a table saw are shown in FIGS. 30-35 .These figures show piston structures configured in different ways andoriented in different directions. In FIG. 30 , a piston rod 600 isshaped to extend around an actuator so the actuator can pull the pistonrod down. In FIGS. 31 and 32 piston rods and actuators are configured sothat the piston rods are pushed away from the actuators.

FIG. 33 shows a configuration for a table saw that has both a main blade401 and a scoring blade 402. Each blade is supported by a somewhat “L”shaped arbor 403 mounted in a saw to pivot around point 404. Each arborincludes an arm 405 and an actuator 406 is operatively connected to arm405 so that when the actuator fires, the piston moves out and pushes arm405. Pushing arm 405 causes the blade to pivot down around point 404,thereby to retract the blade and minimize danger. In this embodiment,two separate actuators are used, one to retract the main blade and theother to retract the scoring blade.

FIG. 34 shows an embodiment where one actuator is used to retract both amain blade and a scoring blade. The actuator is connected to the bladethrough a 4-bar linkage with anchor points in the linkage identified ateach “X”. FIG. 35 shows an embodiment where a single actuator isconnected to two blades by nested and splined shafts. The linkagesbetween the actuators and blades in these embodiments provide astructure to retract the blades in case of danger, while allowing anelevation mechanism to raise and lower the blades to accommodateworkpieces of varying thicknesses.

The fundamental idea of using an actuator and piston assembly asdescribed herein in a safety system for table saws or other power toolsis to move the blade away from the hand of a user faster than the handcan move into the blade. Typically, the speed at which a human canflinch to move a hand is around 2 meters per second. Even assuming aperson's hand could move at a speed of 6 meters per second, an actuatorand piston assembly as described herein can be constructed with a chargesize sufficient to move the blade away faster. An actuator as describedherein can accelerate a piston, blade and arbor support of around 10kilograms at an acceleration on the order of 50 to 200 Gs or more.

One factor in maximizing the effectiveness of an acceleration on theorder of 50 to 200 Gs, or even 50 to 1000 Gs or more, is to minimize thedecay time of the magnetic field so that the acceleration is applied asquickly as possible. One way to minimize the decay time is to chose amagnet material of a higher bulk resistance, for example, stainlesssteel rather than plain steel. The higher resistance increases the ohmicloss and dissipation of eddy currents and thereby reduces the timerequired for the magnetic field to release the cap to allow thepressurized fluid to exit the reservoir. Although stainless steel has ahigher resistance and thereby can be used to minimize the decay time ofthe magnetic field, plain steel can also be used.

Another way to minimize the decay time of the magnetic field is tominimize the thickness of the steel supporting and surrounding coil 52.Minimizing the thickness of the surrounding steel reduces the timerequired for field changes to propagate via eddy current formation anddissipation. Putting a port or channel through the center of the magnet,such as hole 44 in first steel cylinder 43, also reduces the decay timeof the magnetic field by reducing the pole wall thickness while stillallowing a sufficiently large pole area for the cap to seal thereservoir.

The time required to release the pressurized fluid can be shortened byminimizing the inertia associated with the cap. A cap as describedherein is used instead of the piston itself to seal the pressurizedchamber because the cap has significantly less inertia, and therefore,the release of the pressurized fluid can be as fast as possible. Also,minimizing the volume of the space between the cap and the piston bypositioning the bottom of the piston with the top surface of the magnetkeeper minimizes the volume that needs to be filled at activation sothat the piston starts moving as soon as possible.

As mentioned earlier, the distance between the reservoir and the pistonis minimized to thereby minimize the delay in the pressure wave of thereleased fluid of about 1 ms/ft due to the limitation of the speed ofsound. Accordingly, a remote reservoir would release substantially moreslowly.

In an actuator as disclosed herein, operating the magnet close tosaturation (i.e., at a current level where an increase in the currentdoes not produce any significantly greater magnetic force) allows themagnet to be as small as possible. Also, designing the electromagnet sothat it produces a magnetic force sufficient to hold the cap in placeagainst the pressure in the reservoir, but not significantly more, meansthe cap will break away from the electromagnet as soon as possible whenthe current to the electromagnet is turned off, thereby releasing thepressurized fluid as quickly as possible. In some embodiments, theelectromagnet may be designed to produce a holding capacityapproximately 25% greater than the expected pressure in the reservoir,although other amounts may be selected.

Once activated, the piston is accelerated as the pressurized fluid fromthe reservoir pushes against the piston head. At the same time, the airor fluid on the other side of the piston compresses and creates backpressure. The cylinder containing the piston can be sized so that thereis a sufficient volume of air or fluid on the other side of the pistonto slow it down smoothly. The piston will decelerate smoothly if, forexample, the cylinder is long enough for the piston to pass through thepoint of equilibrium where the pressure on one side of the piston headis equal to the pressure on the other side. Alternatively, as mentionedearlier, a vent can be positioned to allow air to escape after thepiston has moved a certain distance so that the piston can deceleratemore smoothly. A bumper can also be used to stop the piston and absorbenergy.

As mentioned earlier, the actuator can be reset after activation by somemechanism such as a spring which draws the piston back to its startingposition. Alternatively, a valve can be placed near one end of thepiston cylinder so that low pressure air can be used to drive the pistonto its original position. An optional breather vent can be installed onthe side of the magnet keeper and connected to a small conduit leadinginto the area beyond the piston. A breather vent is a vent with a filterthat keeps dust and other particles from entering the actuator fromoutside. The conduit and associated hole for the breather vent would besmall enough so that there would be only a negligible amount of leakagethrough the hole as the pressurized fluid is released from thereservoir, but large enough to provide an outlet for air to escape asthe piston is reset to its original position.

FIGS. 36 through 41 show another embodiment of an actuator. The base 802of the actuator, shown in FIG. 36 is a square metal block about 1.5inches thick and about six inches along each side. Three holes 804, 805and 806 are drilled along the front side horizontally into the base,with hole 805 in the middle of the front side and one hole on eitherside of the middle hole. Each of the holes along the front side meets ahole drilled vertically down into the base from the top about an inch infrom the front side, as shown at 808, 810 and 812, so to create threeconduits from the side of the base to the top of the base. Base 802 mayalso include one or more holes at different locations, such as hole 813,to mount the actuator in a saw or other machine. Another hole 814 passesall the way through the center of the base and a groove is cut into theinner surface of hole 814 just below the top surface of the base withinwhich an o-ring 816 is installed. Another circular groove 818 of alarger diameter, wider cut and concentric with hole 814 is carved intothe top surface of base 802. On the bottom surface of groove 818 alongthe inner radius lays another groove for another o-ring 820. Seatedwithin the larger groove 818 and over the o-ring 820 is the base of ashort cylinder 822, as shown in FIG. 37 . Hole 814 has an upper portionthat is of larger diameter than the rest of the hole, and seated withinthe upper portion of hole 814 and within o-ring 816 is a smallercylinder 824 of slightly greater length than outer cylinder 822, asshown in FIG. 38 .

The tops of cylinders 822 and 824 fit within corresponding groove 826and hole 828 on the underside of another metal block 830 called themagnet mount of similar dimensions to the base only thicker, as shown inFIGS. 39 and 40 . Hole 828 has a lower portion of larger diameter inwhich inner cylinder 824 resides. Groove 826 and hole 828 are equippedwith o-rings 832 and 834 in a manner similar to the o-rings in the base.The o-rings help create a seal for the volume enclosed between outercylinder 822 and inner cylinder 824 which forms an air chamber 836similar to air chamber 22 described earlier. Magnet mount 830 isattached to base 802 by four bolts 838, shown in FIGS. 39 and 41 , whichare inserted into holes 840 in the base, one hole located at each of thefour corners of the base. The bolts are inserted from the underside ofthe base and extend vertically upward where they are then screwed intothreaded holes 842 in the magnet mount. Three large holes 844 passthrough the magnet mount from the top surface vertically downwardopening into air chamber 836. A groove 846 is carved along the top edgeof each hole 844 to hold an o-ring and to create a shelf or ledge aroundthe perimeter of each hole to support a magnet as will be seen later.FIG. 42 shows a printed circuit board 848 placed on top of magnet mount830. Circuit board 848 is shown isolated in FIGS. 43 and 44 . Four holes850, align above holes 842 in the magnet mount and a hole 852 located atthe center of the board aligns above hole 828 in the magnet mount. Anine pin D-sub connector 854 is mounted on the bottom of the circuitboard along, and at the middle, of the front edge of the board such thatthe connector projects out from the front edge of the board. The D-subconnector fits within an area 856 carved out along the top and frontedge of the magnet mount so that the circuit board may lie flat againstthe top surface of the magnet mount.

Two tactile or tact switches 858 are mounted to the circuit board, onetowards the right rear corner and the other towards the left rear.Electrical traces run from each tact switch to the D-sub connector, asshown in FIG. 44 . The tact switches are positioned so that when apiston is fully lowered or retracted the piston compresses the tactswitches. In this manner the tact switches are used to indicate to acontrol system the presence of a piston and that the piston is in theproper position for firing.

Circuit board 848 also has three large holes 860 that are shapedgenerally like a circle with a small rectangle along the side. A magnetsimilar to the magnet described earlier fits into each of the holes sothat the actuator has three magnets, each magnet with its own cap. Twoelectrical contacts 862 are provided for each magnet, one for each endof the magnet coil, and traces run from each contact to pins on theD-sub connector to supply power to the magnets, as seen in FIG. 44 .

As shown in FIG. 45 , a piece of foam 864 is placed over the circuitboard and has cutouts for the magnets, tact switches, screws and otherholes so that the foam lies flat. Foam 864 is shown isolated in FIG. 46. The foam prevents dust and other contaminants from entering theactuator.

As shown in FIG. 47 , adjustable plungers 866, shown isolated in FIGS.48 and 49 , are positioned over the tact switches, one sitting on top ofeach tact switch. The adjustable plungers provide a mechanism for thepiston to depress each switch as the piston moves down. The adjustableplungers have vertically oriented set screws 868 whose height can beadjusted. The plungers are made of a material such as plastic so thatwhen a piston retracts, the piston contacts the set screws and pushesthe plunger against the tact switches. The height of each set screw isadjustable so that each screw can be set to contact its tact switch asdesired. The plungers are held in position by a cap retainer describedbelow.

Two plastic caps 870 are placed on the surface of the circuit board overthe bottoms of the screws that attach the D-sub connector to the circuitboard. The plastic caps protrude up above the circuit board toelectrically isolate the screws so that they do not make contact withother parts of the actuator.

Also shown in FIG. 47 are three magnets and cap assemblies 872 installedin the actuator. Magnet and cap assemblies 872 are shown connected tocircuit board 848 in FIG. 50 and isolated in FIG. 51 . The upper part ofeach magnet is of a larger diameter than the rest of the magnet and thiscreates a lip that sits on the shelf or ledge created by groove 846around the top of each of the large holes 844 in the magnet mount. Asshown in FIG. 51 , each magnet has a plastic molded extension 874attached along the side at the top of the magnet. Extension 874 attachesto the side of the magnet housing by a screw 908 which passes through ahole 910 along the front of extension 874 and into a hole 912 on theside of the magnet housing, as shown in FIGS. 51 and 52 . Eachextension, in turn, has electrical contacts 876 embedded in the plastic,and holes 878 extend through the plastic so that the two ends of themagnet coil can extend out and connect to the electrical contacts 876.The electrical contacts are then attached to the circuit board by screws880, as shown in FIG. 50 . Cavities 882, as well as cavity 856, carveddown into the top surface of the magnet mount provide clearance for theends of the screws to extend beyond the bottom of the printed circuitboard without touching the magnet mount. As mentioned earlier,electrical traces or power planes connect each screw to a pin on theD-sub connector and supply power to the magnets.

FIG. 52 shows an exploded view of magnet and cap assembly 872. Themagnet includes an outer cylindrical housing 888 and an innercylindrical insert 890. The housing and inserts are shown together inFIGS. 53-55 and shown in cross-section in FIG. 55 . Outer housing 888 isin the shape of a cylinder enclosed at the bottom except for a hole 891in the bottom center. Near the top a section 892 of the cylinder has alarger outer diameter than the rest of the cylinder while maintainingthe same inner diameter so that there is a shelf created around the topof the housing which is used to mount the magnet, as explained. Section892 is positioned slightly below the top of the housing so that the topsurface 894 of the housing is of the same radial thickness as thehousing below section 892. Top surface 894 forms the magnetic pole towhich the magnet cap is attracted. Keeping the radial thickness of topsurface 894 the same as the housing below section 892 helps minimize thetime for the magnetic field at the top surface to decay, therebyreleasing the cap as quickly as possible, as explained previously.

Insert 890 is shaped like a cylinder 895 with a shoulder 896 toward thebottom. Shoulder 896 includes a groove on the underside in which ano-ring 900 is installed. A threaded section 898 extends below shoulder896. The insert 890 is inserted into the housing 888 and then a nut 902shaped like a threaded ring is screwed on the threaded end to hold theassembly together. A coil of wire is wound around insert 890 and housedin the gap between insert 890 and housing 888. Two holes 904 on the sideof housing 888 extend through section 892 to allow each end of the coilto exit the housing. The wires pass through holds 878 in plastic moldedextension 874 and connect to electrical contacts 876, as explained.

FIGS. 56 through 60 show an alternative embodiment for a magnet. In thealternative embodiment insert 890 is replaced by the insert 914 andhousing 888 is replaced by housing 916. Insert 914 is similar to insert890 except that it is not threaded at the bottom. As shown in thecross-sectional view in FIG. 59 , the hole 891 at the bottom of housing916 flares outward toward the bottom of the housing. Insert 914 isinserted into housing 916 and, once in place, a tool is used to flarethe end 918 of the insert so that it matches the flare around hole 891in the housing, as shown in FIGS. 57, 59 and 60 .

FIGS. 52 and 61 show a cap 886 similar to the cap discussed earlierexcept cap 886 includes a contact disk 920. Contact disk 920 is shownisolated in FIG. 62 . A screw 922 passes through a hole in the center ofthe contact disk to connect it to the top of the cap and to connect thecap to guide 924. The contact disk provides a raised surface to impactthe bottom of the piston head when the actuator fires. By striking thepiston head, the contact disk imparts energy to the piston to start thepiston moving as quickly as possible. Additionally, transferring energyfrom the cap to the piston head slows the cap down so that when the capreaches the limit of its travel by contacting fingers 102, as describedearlier, the force of the contact between the cap and the fingers islessened and the likelihood of bending the fingers or damaging the capis decreased. Contact disk 920 is made of a hard plastic or some otherhard material, although the material is softer than the piston head toprevent damage to the piston head. Holes are punched through the contactdisk to minimize its mass so that the cap can accelerate as quickly aspossible. In the depicted embodiment, the cap is approximately 118th ofan inch thick (˜3 mm) steel to provide material for the magnetic field,and the mass of the cap is approximately 23 grams. With fins, the cap isapproximately 25 grams.

With the magnets installed, another metal block or plate called a capretainer 934 is placed flat against the magnets and sits on the magnets,as shown in FIG. 63 , to hold the magnets down and stop the caps. Capretainer 934 has similar dimensions to base 802 but is thinner. Capretainer 934 has four holes 936, one at each corner, which align withholes 842 in the magnet mount and a hole 938 in the center that alignswith hole 828 in the magnet mount. Nine socket head cap screws 940attach the cap retainer to the magnet mount. Screws 940 pass throughholes 942 in the cap retainer, shown in FIG. 64 , and each hole 942 hasa section of smaller diameter to catch the head of the screw. The screwsthen pass through corresponding holes in the foam and in the circuitboard and then thread into threaded holes 948 in the magnet mount. Asshown in FIG. 65 , the underside of cap retainer 934 has cutouts 950 forthe magnet assemblies 872 and cutouts 952 for the adjustable plungers sothat the bottom surface of the cap retainer can lie flat against themagnets. Cutouts 952 are shaped around the adjustable plungers tocapture the adjustable plungers and hold them in place and keep themfrom rotating when the screws are adjusted. Two small holes 954 in thecap retainer allow the heads of the set screws to protrude slightlyabove the top surface of the cap retainer, as shown in FIG. 63 , so thatthe piston head can contact the plungers. The cap retainer has threelarge holes 956, one over each magnet, with six fingers 958 around thecircumference of each hole. The ends of the fingers curl and extend overthe outer edge of each magnet to stop the cap, as described above withfingers 102. A very small hole 960 is drilled into the cap retainer andextends down from the top surface but does not go down to the bottomsurface. Instead, it meets the path of another hole 962 drilledhorizontally in the side of the cap retainer in which is installed abreather vent 964, as shown in FIG. 63 . The purpose of the breathervent is described below.

There is a circular groove 968 on the top surface of the cap retainerand an o-ring 970 fits in the bottom of the groove. One end of a largecylinder 972 also fits into the groove 968, as shown in FIG. 66 , and ontop of o-ring 970 to create an airtight seal with the cylinder.

A top metal plate 974 of similar dimension to the base only thinner andwith a similar groove and O-ring fits over the top of the largecylinder. Four long bolts 976 secure the large cylinder and the rest ofthe assembly together. Each bolt 976 passes through a hole, one locatedin each corner of top plate 974, and then through holes 936 in the capretainer, then through corresponding holes in the foam and circuit boardand into holes 842 in the magnet mount. Holes 842 in the magnet mountalso receive the ends of bolts 838 coming up from the base. Threadedends of bolts 838 are hollow with additional threads on the innersurface, as shown in FIG. 41 , so that the threaded ends of long bolts976 thread into the threaded holes 978 in bolts 838. Top plate 974 alsohas three holes along the front side, a center hole 980 and two holes oneither side of the center hole, similar to the three holes on the frontside of the base. Each of the three holes is connected to holes that aredrilled vertically from in the underside of the top plate thus creatingthree separate conduits into the area enclosed by large cylinder 972.The center hole 805 along the front side of the base and the center hole980 along the front side of the top plate are used for connecting a hosefrom a high pressure fluid source. Hole 805 in the base empties into airchamber 836 and is used to fill air chamber 836 with high pressure airor fluid. Likewise, hole 980 in the top plate empties into the area inthe large cylinder above the piston head and may be used to release orfill, or release at one time and fill at another, this area with air orfluid to help, for example, decelerate the piston by creating anadjustable air dampener. The two holes 806 along the front side of thebase and the two holes along the front side of the top plate on eitherside of the center hole are used to allow a set of pressure sensors 982to measure the pressure in air chamber 836 and in the large cylinderabove the piston head. Only one pressure sensor is needed for each areabut a second one is added for redundancy. Information from the pressuresensors is sent to a controller. Breather vent 964, which was mentionedearlier, is also used to help decelerate the piston smoothly by allowingair to escape from underneath the piston after the actuator has fired.The hole 960 in the cap retainer is small enough that it will providevirtually no leak at all when the high pressure air is released becausethe air cannot escape very quickly. But after the initial release, someair may gradually escape through the breather vent to allow the pistonto return to its original or ready position.

A piston 984 is shown in FIGS. 67 and 68 . The shaft 985 of the pistonpasses through center holes in the cap retainer, the foam layer, thecircuit board, the magnet mount (which is lined with an o-ring), themiddle of small cylinder 824, and finally through center hole 814 in thebase (which is also lined with an o-ring). As shown in FIGS. 69 through71 , piston shaft 985 then passes through a hole 988 in a fitting 986that is attached to the bottom of base 802 by four screws 987. Fitting986 encases a 989 which is flush with the top surface of the fitting, asshown in FIG. 71 . The bushing fits around the piston shaft to reducefriction as the piston moves. (A bushing may also be used in hole 938 inthe cap retainer.) Towards the bottom of fitting 986, a wiper 990 madeof an elastomeric material is installed within the fitting and aroundthe shaft of the piston, as shown in FIG. 72 . The wiper extends towardsthe shaft at an angle to help keep dust out of the actuator by rubbingagainst the piston shaft as the shaft moves up and down.

FIG. 73 shows piston 984 isolated and FIG. 74 shows an exploded view ofthe piston. Piston 984 consists of a shaft 985 which is attached to apiston head 991. Shaft 985 has a short section of smaller diameter atthe end near the piston head which is threaded. Piston head 991 has theshape of a circular disk, as shown in FIG. 75 , with a hole 992 in thecenter. Hole 992 has an upper section of a larger diameter, a middlesection of a smaller diameter and a lower section of an even smallerdiameter. A knob 993, shown in FIGS. 74 and 76 , consists of an uppercircular portion on top of a lower cylinder that is of a smallerdiameter. A threaded hole 994 passes through both sections. Knob 993fits into hole 992 in the center of the piston head such that the bottomsurface of the upper portion of the knob rests on the surface at thebottom of the upper section of hole 992. The bottom of knob 993 restsupon the surface at the bottom of the middle section of hole 992. Shaft985 passes through the bottom section of hole 992 and threads into hole994 of knob 993 to hold the piston assembly together. As shown in FIG.75 , piston head 991 has generally triangular sections 995 cut out ofthe top surface to decrease the overall weight of the piston headwithout compromising structural support. An o-ring 996 fits into agroove around the outside edge of the piston.

FIG. 77 shows a schematic drawing of a system to provide pressurized airto a reservoir in an actuator. An air supply is identified at 1002, andair from that supply passes through line 1003 to a blow-off valve 1004,which may be set to operate at a preselected pressure, such as 150pounds per square inch (psi) or 1 megapascal (MPa). The air then passesthrough a particle filter 1006 to a pressure regulator 1008, and then toa three-way joint or tee 1010 in the line. Branch 1012 from the teechannels air to a pressure booster 1014 which boosts the pressure to thedesired level, which for the actuator embodiments described herein willbe around 300 psi (2 MPa), or more generally, 200 to 400 psi (1.4 to 2.8MPa). A two-way valve 1016, which is normally closed, is downstream fromthe booster and that valve opens to supply pressurized air to theactuator through a particle filter 1018. Valve 1016 may be opened andclosed by a control system that receives a signal from one or morepressure sensors, such as pressure transducers 1020, that measure thepressure within the actuator's reservoir.

A tee 1022 is positioned between valve 1016 and filter 1018 and branch1023 from the tee leads to a two-way valve 1024 that is normally openand from there to a particle filter 1026 and then to atmosphere. Thisbranch provides a path for pressurized air to exit the actuator, or inother words, provides a path to bleed-off air from the reservoir. Valve1024 can be controlled by any appropriate control system.

A branch 1030 from tee 1010, which is upstream from booster 1014,provides air to a regulator 1032 and from there to a blow-off valve1034, which may be set to operate at a preselected pressure, such as 100psi (0.7 MPa). The air then passes through a two-way valve 1036 which isnormally closed, and from there through a particle filter 1038 to thepiston cylinder in the actuator. The air passing through valve 1036 issupplied to the cylinder on the side of the piston opposite thereservoir of pressurized gas or air. By providing air to this side ofthe piston head, the piston can be reset, as described previously. Valve1036 can be controlled by any appropriate control system, and such acontrol system may include pressure switches, such as switches 1040.

As alternatives to the system described above in which booster 1014provides pressurized air to the actuator, either a compressor or acylinder or container of compressed air (or some other appropriate gas)can supply pressurized air directly to valve 1016 and also to regulator1032. The components of the system downstream from valve 1016 andregulator 1032 would remain the same in the alternative embodiments. Inembodiments using a compressor or cylinder of compressed air, a dryercan be inserted into the system between the compressor or cylinder andvalve 1016 and regulator 1032 to dry the air, and other components suchas blow-off valves can be used as needed.

An embodiment with a piston head having an area of 16 in² (0.01 m²), andusing pressurized air at 300 to 350 pounds per square inch (˜2 MPa),results in 4,800 to 5,600 pounds of initial force (˜21,000 to 25,000 N).Such an embodiment, therefore, will accelerate a 20 kg mass at roughly1,000 m/s², which is on the order of 100 Gs. It is believed that with aforce applied within 0 to 3 milliseconds, accelerations of 20 to 500 Gsare possible and effective.

As explained previously, actuators as described herein can be cycledrepeatedly, and that, in turn, allows a retraction mechanism in amachine such as a table saw to be cycled for self-testing or for otherreasons. It is conceivable that a mechanical structure such as aretraction mechanism could jam or freeze or become blocked. Accordingly,to ensure the retraction mechanism is operable (i.e., free to move),and/or to ensure the actuator is working, the actuator and retractionmechanism can be cycled after a predetermined period of time or beforeor after specific operations. Tests involving cycling of an actuatorand/or a retraction mechanism may be performed, for example, upon powerup and upon power down of a machine, before or after specificoperations, or whenever a user wants to ensure that the actuator and/orany associated retraction mechanism is not frozen or jammed. A switch orother user input device may be provided to allow a user to volitionallycycle the actuator and/or retraction mechanism. A cycle of a retractionmechanism in a table saw may be considered as retracting the blade belowthe table and then moving the blade back up above the table, simplyretracting the blade downwardly, or simply moving the blade or some partof the retraction mechanism a small distance to make sure the retractionmechanism is not jammed or frozen.

FIG. 78 shows a flowchart illustrating various paths of cycling aretraction mechanism in a woodworking machine. Block 1050 represents oneor more operations that may be performed in a cycle. The specific blockswithin block 1050 form an example set of operations that may compose acycle. Block 1052 represents various functions that may be performed tocheck the system such as gathering measurements of pressure levels,piston location, blade presence, and so on. Block 1054 representstriggering the actuator and retraction mechanism. Block 1056 representsre-setting the actuator and restoring the retraction mechanism to itsnormal operating position. Block 1058 represents another system checkwherein the readiness of the safety system to retract the blade again isconfirmed. Block 1060 represents a system ready state that the systemwould stay in under normal operating conditions, where the safetysystem, including the actuator, is ready to fire the moment a safetyhazard is detected. Blocks 1062, 1064 and 1066 represent differentconditions that may trigger a cycle to be preformed such as a period oftime that the system may wait before cycling, a set of operations thatmight be performed before cycling or a command to power down that mighttrigger a cycling to occur. The cycling paths in FIG. 78 are onlyexamples and the system is not limited to the possibilities shown.

Testing to see if the retraction mechanism is frozen or jammed couldalso be accomplished by moving at least a portion of the retractionmechanism by hand, by triggering a single use actuator that isreplaceable (such as an explosive charge), by moving an elevationcontrol wheel or handle that is or can be linked to the retractionmechanism, or by some other method. For example, a machine may include acontrol system with a sensor or switch to detect motion of theretraction mechanism or a part thereof. The control system may requirethat the sensor or switch detect motion of the retraction mechanismperiodically or at predetermined times, and if not, then the controlsystem could prevent the machine from operating or being turned on. Auser could move the retraction method by hand or by turning a hand-wheelwhen required by the control system.

Whether the retraction mechanism is tested by cycling an actuator or bymoving at least a part of the retraction mechanism by hand or in someother way, what is important is to make sure the retraction mechanism isfunctional and able to move to retract the blade in the event of anaccident.

As discussed previously, one of the fundamental ideas of using anactuator and piston assembly as described herein in a safety system fortable saws or other power tools is to move the blade away from the handof a user faster than the hand can move into the blade. As stated,typically the speed at which a human can flinch to move a hand is around2 meters per second, and even assuming a person's hand could move at aspeed of 6 meters per second, an actuator and piston assembly asdescribed herein can be constructed to move the blade away faster. Forexample, an actuator as described herein can apply at least 4,000 poundsof force within less than 600 microseconds and easily within 1millisecond after the electromagnet in the actuator is triggered.

The force applied by the actuator will quickly accelerate the pistonassembly and blade and cause the blade to retract, as explained. Oncethe blade is retracted, however, the blade's movement must be brought toa stop. The blade will develop significant energy as it is acceleratedby the actuator, and that energy must be dissipated to stop the blade.As explained previously, a bumper can be used to stop the piston andabsorb energy; however some bumpers, such as rubber bumpers, might causeor allow the blade to rebound or bounce back up. Another way of stoppingthe blade while avoiding rebound is to use a stopping mechanism such asa cylinder filled with oil or other fluid that operates like a damper orcatch. FIG. 79 shows an oil cylinder catch 1080 composed of a cylinder1082 filled with oil and a piston 1084 encased in the cylinder. Amovable top 1086 is attached to one end of rod 1088 connected to piston1084 and the top is biased away from the cylinder by a spring 1090.Spring 1090 functions to extend piston 1084 and rod 1088 after aretraction so that the catch is ready for a subsequent retraction; thespring is not strong enough to cause a rebound of the blade andsupporting structure during a retraction. Mounting holes 1092, onelocated in the movable top and one at the other end of the cylinder,allow the device to be mounted in a machine, as shown symbolically inFIG. 80 , where one end is mounted to the saw and the other to an impactblock 1094.

As blade 400 retracts, structure supporting the blade strikes impactblock 1094, forcing piston 1084 to move downward against the volume ofoil in cylinder 1082. Orifices, which may be located on the piston or inan inner cylinder, allow the oil to move out of the region below thepiston and are sized and positioned according to the desireddeceleration. The oil slows the downward movement of piston 1084 andgenerates an upward force on impact block 1094 to decelerate the blade,as shown by the arrow labeled “F” in FIG. 80 . In this manner, thecylinder and piston counteract the downward movement of the blade andinelastically absorb the energy associated with the blade's downwardmovement to stop the blade without rebound.

An advantage of an oil cylinder catch is that it can provide a generallyconstant or level deceleration as the piston moves down against thefluid; in other words, the force decelerating the blade does not varysubstantially during deceleration as it would with a spring or othernon-constant force applicator. The force that the oil cylinder catch orother stopping mechanism needs to withstand depends on the mass,velocity and deceleration of the blade and associated support structure,such as an arbor support. By way of example, an oil cylinder catch asdepicted may provide a level 4,500 pounds of force (˜20,000 Newtons)over a piston movement of around 1 to 6 inches to decelerate a 20 kgmass moving at a velocity on the order of 10 m/s. An oil cylinder catchprovides a compact solution that can withstand a large force of impactand that reduces or eliminates rebound. Additionally, an oil cylindercatch could decelerate the blade arbor support over a longer distancethan a rubber stop, thereby reducing the stress on the structuresupporting the blade. This is important because stopping a relativelyheavy, fast moving blade and support structure over a relatively shortdistance could damage bearings and/or bend shafts or other parts used inthe support structure. Additionally, providing a substantially constantor level deceleration allows for the maximum acceptable decelerationforce (i.e., the maximum force that will not damage machine componentssuch as bearings and shafts) to be applied over the longest timepossible. Of course, although a generally constant or level decelerationhas advantages, it is not required and a piston and cylinder can bedesigned to provide an increasing or decreasing force.

FIG. 80 shows the oil cylinder catch oriented vertically but it may havevarious orientations and linkages. FIG. 80 also shows the oil cylindercatch used with an actuator that pulls blade 400 down, but the oilcylinder catch can be used equally effectively with an actuator thatpushes or otherwise moves the blade.

FIGS. 81 and 82 show a circuit similar to the circuit discussedpreviously in connection with FIG. 23 . The circuit shown in FIGS. 81and 82 may be used to control the current through the coil of anelectromagnet. In this circuit a voltage source V is connected to thecoil L which is in turn connected to a switch labeled SW1 at the nodelabeled N. The other side of switch SW1 is connected to ground. A zenerdiode Z is connected between ground and node N with the cathode at nodeN. The cathode of a diode D is connected at node N and the anode ofdiode D is connected to the terminal of another switch SW2. The otherterminal of switch SW2 is connected to a capacitor C the other side ofwhich is connected to ground. To turn the electromagnet on, that is, toenergize coil L, switch SW1 is closed while switch SW2 is left open sothat the coil L is connected to the voltage source V at one end and toground at the other end which establishes a current IF through the coil.Diode D prevents current from flowing towards the capacitor. Currentpassing through a coil creates a magnetic field about the coil and inmetal within or surrounding the coil, thus turning the electromagnet on.To turn off the electromagnet, switch SW1 is opened, which causes a veryhigh back emf voltage spike that is limited or clamped by the zenerdiode Z. The zener diode limits the voltage enough to protect thecircuit from damage but still allows the voltage to be very high. Thereis a direct relationship between how fast the current changes and howhigh the voltage will spike. Theoretically, the voltage spike would beinfinite if the circuit is broken instantaneously. Creating a high backemf voltage generates large eddy currents in the surrounding metal whichwill dissipate the energy in the magnetic field more quickly due tomaximizing ohmic losses from eddy currents. After switch SW1 in opened,current passing through the coil flows into the cathode of the zenerdiode Z to ground. In addition to using a high back emf voltage toenable the magnet to turn off quickly, a reverse pulse is applied to thecoil L by capacitor C which is charged to a high voltage no higher thanthe voltage limit set by the zener diode Z so that the current from thecapacitor flows into the coil L and not into the zener diode Z. Thereverse pulse works to collapse the magnetic field more quickly due tothe larger eddy currents that will occur in the surrounding metal to tryand oppose changes in the magnetic field. To apply the reverse pulse,switch SW2 is closed just after switch SW1 is opened, as shown in FIG.82 , and a reverse current IR flows through the coil L which helps tocollapse the magnetic field as rapidly as possible.

FIGS. 84 through 95 show an embodiment of a circuit for controlling anelectromagnet with a microprocessor. FIG. 83 gives a high-level view ofthe circuit showing how the circuit portions shown in FIGS. 84 through95 fit together. FIG. 84 shows power supply circuits, FIG. 85 showsconnectors, FIG. 86 shows circuitry involved with releasing or turningoff the electromagnet, FIGS. 87 and 88 show circuitry involved withgenerating a reverse pulse, FIGS. 89 and 90 show circuitry involved withthe current through the coil, FIG. 91 shows a microprocessor, FIG. 92shows microprocessor support circuitry, FIG. 93 shows bus transceivercircuitry, and FIGS. 94 and 95 show current amplifier circuitry.

FIG. 96 shows a simplified diagram of the portion of the circuit shownin detail in FIGS. 88, 89 and 91 that controls the current through thecoil of an electromagnet. The microprocessor turns on the coil byclosing switch SW_RELEASE with switch SW_REVERSE left open, whichconnects one end of the coil to ground and closes switch SWH whichconnects the other end of the coil to twenty four volts therebyestablishing a current through the coil. To turn off the magnet, themicroprocessor opens switch SW_RELEASE which causes a back emf voltagethat is limited to a high voltage by the zener diode ZENER_CLAMP throughwhich the current through the coil flows to ground. Diode DIODE preventscurrent from the back emf spike from flowing toward the capacitor. Themicroprocessor closes switch SW_REVERSE which allows the capacitorCAPACITOR RELEASE_PULSE which is charged to a high voltage to dischargethrough diode DIODE and switch SW_REVERSE. The microprocessor alsocloses switch SWL to open a path for the current to ground, although thecurrent may be allowed to flow back through the twenty four volt powersupply if the power supply is equipped to handle the current.

FIG. 96 illustrates a circuit used to control one electromagnet, but itcan easily be expanded to service more electromagnets, as shown in FIG.97 . In the circuit shown in FIG. 97 , the microprocessor has three setsof control lines to service three separate electromagnet coils for threeelectromagnets. Although there are three release switches, one for eachcoil, there is only one switch, SW_REVERSE, to activate the reversevoltage from the capacitor. That switch, which may be implemented by asilicon controlled rectifier, is chosen by the amount of current to behandled, which is greater in the circuit with three coils than in thecircuit with one coil.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to fast-acting and low-inertiaactuators which may be useable in various applications where a highforce must be applied very quickly, such as in safety systems for powertools. The present disclosure is particularly applicable to the powertool industry and to woodworking machines and other similar machines.

It is believed that this disclosure encompasses multiple distinctinventions with independent utility. While each of these inventions hasbeen disclosed in its preferred form, the specific embodiments thereofas disclosed and illustrated herein are not to be considered in alimiting sense as numerous variations are possible. The subject matterof the inventions includes all novel and non-obvious combinations andsub-combinations of the various elements, features, functions and/orproperties disclosed herein. No single feature, function, element orproperty of the disclosed embodiments is essential to all of thedisclosed inventions. Similarly, where the claims recite “a” or “afirst” element or the equivalent thereof, such claims should beunderstood to include incorporation of one or more such elements,neither requiring nor excluding two or more such elements.

It is believed that the following claims particularly point out certaincombinations and sub-combinations that are directed to one or more ofthe disclosed inventions. Inventions embodied in other combinations andsub-combinations of features, functions, elements and/or properties maybe claimed through amendment of the present claims or throughpresentation of new claims. Such amended or new claims, whether they aredirected to different inventions or directed to the same inventions,whether different, broader, narrower or equal in scope to the originalclaims, are also regarded as included within the subject matter of theinventions of the present disclosure.

The invention claimed is:
 1. A power tool comprising: a blade forcutting workpieces; a motor configured to drive the blade; a structuresupporting the blade and configured to allow the blade to retract; andat least one electromagnet to trigger a retraction of the blade, and acircuit that controls the at least one electromagnet, where the circuitis configured to supply current to the at least one electromagnet in afirst direction to create a magnetic field and in a second direction toreverse the magnetic field.
 2. The power tool of claim 1, where thecircuit that controls the at least one electromagnet with amicroprocessor.
 3. A power tool comprising: a blade for cuttingworkpieces; a motor configured to drive the blade; a structuresupporting the blade and configured to allow the blade to retract; atleast one electromagnet to trigger a retraction of the blade; and acircuit that controls the at least one electromagnet with amicroprocessor, where the circuit is configured to supply current to theat least one electromagnet in a first direction to create a magneticfield and in a second direction to help dissipate the magnetic field. 4.The power tool of claim 1, 2, or 3, further comprising a detectionsystem configured to detect a dangerous condition between a person andthe cutting tool, and where the at least one electromagnet triggers theretraction of the blade upon detection of the dangerous condition.
 5. Apower tool comprising: a blade for cutting workpieces; a motorconfigured to drive the blade; a structure supporting the blade andconfigured to allow the blade to retract; at least one electromagnet tocontrol retraction of the blade, and a circuit that controls the atleast one electromagnet, where the circuit is configured to supplycurrent to the at least one electromagnet in a first direction to createa magnetic field and in a second direction opposite the first direction.6. The power tool of claim 5, where the circuit controls the at leastone electromagnet with a microprocessor.
 7. A power tool comprising: ablade for cutting workpieces; a motor configured to drive the blade; astructure supporting the blade and configured to allow the blade toretract; at least one electromagnet to control retraction of the blade;and a circuit that manages the at least one electromagnet with amicroprocessor, where the circuit is configured to supply current to theat least one electromagnet in a first direction to create a magneticfield and in a second direction to help dissipate the magnetic field. 8.The power tool of claim 5, 6, or 7, further comprising a detectionsystem configured to detect a dangerous condition between a person andthe cutting tool, and where the at least one electromagnet causes theblade to retract upon detection of the dangerous condition.